1. Field of Invention
The present invention relates to a hydraulic control system for an automatic transmission. More specifically, the present invention relates to a hydraulic control system for an automatic transmission, which cuts supply of the source pressure used for engaging a brake in a lock-up stage where an input shaft and an output shaft are directly connected.
2. Description of Related Art
In a conventional hydraulic control system for an automatic transmission (hereinafter simply “hydraulic control system”) a plurality of shift valves are selectively changed by shift solenoid valves responsive to operation of a shift lever. Such a hydraulic control system includes a plurality of friction engagement elements including clutches and brakes. The friction engagement elements corresponding to the respective shift speeds may be engaged/disengaged to change the shift stage so as to establish the required speed.
In the conventional automatic transmission, shifting may be performed by engaging one friction engagement element, while disengaging another of the friction engagement elements, i.e., a clutch-to-clutch shift. In this case, linear solenoid valves and control valves corresponding to the respective friction engagement elements supply regulated engagement pressures to the hydraulic servos which operate the respective friction engagement elements to effect the aforementioned shift. However, the conventional transmission requires a large number of relatively large size linear solenoid valves. Accordingly, the structure of the conventional hydraulic control system is large, complicated and costly.
The size and cost of the hydraulic control system may be reduced by reducing the number of linear solenoid valves and control valves, that have been provided for the respective hydraulic servos of a plurality of friction engagement elements, to a single pair. The resultant structure of the hydraulic control system is thus simplified and, accordingly, a cost reduction is realized.
Referring to the hydraulic control system for an automatic transmission 10 as shown in FIG. 1, in the 5th speed, for example, the aforementioned single pair of a linear solenoid valve and a control valve are operated to supply the engagement pressure in the 1st to 3rd shift speeds and 5th speed wherein the brakes are engaged as shown in FIG. 2. They are also operated to cutoff supply of the engagement pressure and thereby disengage the brakes in 4th speed where the input shaft and the output shaft of the automatic transmission are directly connected. However, it is necessary to provide for the possibility that a failure may occur in those valves such as by valve sticking. Provision for such a failure may complicate the structure of the circuit, and also complicate any modification aimed at reduction of the size of the hydraulic control system.